Coupling band structure and oxidation-reduction potential to expound photodegradation performance difference of biochar-derived dissolved black carbon for organic pollutants under light irradiation

Herein, the photodegradation performances difference of rice straw biochar-derived dissolved black carbon (DBC) for Tetracycline and Methylene Blue under visible light irradiation have been investigated. Tetracycline is easier degraded (degradation rate: 68%), followed by Methylene Blue (degradation rate: 14%). Singlet oxygen (O-1(2)), superoxide radicals (center dot O-2(-)), holes (h(+)) and triplet DBC ((DBC)-D-3*) are all make contribution for Tetracycline degradation by DBC, whereas just singlet oxygen (O-1(2)), superoxide radicals (center dot O-2(-)) and (DBC)-D-3* are involved in the MB degradation by DBC. Singlet oxygen (O-1(2)) maybe from the fulvic acid-like structure of DBC, while band structure of DBC can explain why superoxide radicals (center dot O-2(-)) and holes (h(+)) can be formed, whereas hydroxyl radicals (center dot OH) cannot be formed. The oxidation-reduction potential results of Tetracycline and Methylene Blue suggests that Tetracycline is easier to be oxidized than Methylene Blue as well as Methylene Blue is easier to be reduced than Tetracycline. Furthermore, experimental and theoretical results support that DBC has good interaction with Tetracycline, but the interaction between DBC and Methylene Blue is very weak. This likely explain why holes (h(+)) is not detected for Methylene Blue degradation by DBC since Methylene Blue has not too much chance to meet holes (h(+)). TC photodegradation intermediates are less toxic than Tetracycline based on QSAR method. Two possible photodegradation path of Tetracycline by DBC are proposed according to HPLC-MS results.